In all forms of lightwave communications systems, noise from a variety of causes can interfere with the users' communications. Examples of corrupting noise include noise originating as part of the transmitted signal, noise created in the process of digital encoding and signal formatting, noise introduced by crosstalk in couplers or reflections in optical components, noise caused by the distortion of the signal or creation of unwanted harmonic products due to nonlinearities at the transmitter or receiver, quantum or modal noise within the optical source, and so forth.
The presence of noise in lightwave communications systems is annoying or distracting to users, can adversely affect speech quality, can reduce the performance of speech coding and speech recognition apparatus, and in some instances completely render the optical information signal unintelligible to the system and thus the user. As a consequence, there exists a need to filter such noise from an optical information signal.
Lightwave communications systems involving optical amplifiers have a particular problem with amplified spontaneous emission (ASE) noise. ASE noise arises in the amplification process. The resonant medium that provides amplification by the process of stimulated emission also generates spontaneous emission. The light arising from spontaneous emission is independent of the input to the amplifier, and represents a fundamental source of amplifier noise. Whereas the amplified signal has a specific frequency, direction, and polarization, the ASE noise is broadband, multidirectional, and unpolarized. As a consequence it is possible to filter out some of this noise by following the amplifier with a narrow bandpass optical filter.
Fixed optical filters such as multilayer dielectric coatings can be used to filter out ASE noise. Properties of such fixed optical filters are discussed, for example, in a book edited by Walter G. Driscoll and William Vaughn, titled Handbook of Optics, Chapter 8, Mcgraw-Hill, New York, 1978.
There are many problems, however, associated with the use of fixed optical filters. For example, fixed optical filters require precise wavelength matching of the filter and the lightwave signal. This requirement increases the cost of the filter and the entire system. Furthermore, light sources are susceptible to aging, leading to a mismatch of the lightwave signal to the fixed filter over time, and necessitating repair or replacement. Fixed filters are also sensitive to environmental changes. Consequently, the environmental conditions surrounding the filter must be carefully controlled and monitored. Environmental changes such as temperature fluctuations can distort the filter wavelength thus requiring repair or replacement. In addition, if the lightwave communications system is upgraded and uses a different or additional wavelength for the transmission signal, the filter must be replaced to match the new wavelength. For those systems using multi-wavelength signals, a filter is required for each wavelength, greatly increasing the initial and replacement costs associated with each filter.
Another type of filter capable of filtering out ASE noise is a refractive-index filter. Examples of refractive-index filters are discussed in "Transient Bragg reflection gratings in erbium-doped fiber amplifiers," by S. J. Frisken, Optics Letters, Vol. 17, No. 24, Dec. 15, 1992, and "Nonlinear wave mixing and induced gratings in erbium-doped fiber amplifiers," by Baruch Fischer, Optics Letters, Vol. 18, No. 24, Dec. 15, 1993.
Tunable refractive-index filters such as those discussed in Frisken and Fischer induce a refractive-index grating in a doped medium to reflect an optical information signal to an output port while unwanted noise is passed through the filter to be absorbed elsewhere. Refractive-index filters split a control wave using an optical coupler. These filters then direct a beam through each end of a doped medium, such as erbium doped (Er-doped) fiber. Propagating the beams in opposite directions creates a standing wave, which in turn induces a refractive-index grating capable of reflecting certain wavelengths of light. By carefully controlling the refractive-index grating, the wavelengths representing noise can be separated or "filtered" from the signal wave.
These filters, however, are problematic. The refractive-index grating merely re-routes the unwanted noise without absorbing it, so care must be taken to avoid leakage of the noise back into the communication system. In addition, these filters require the use of both a pump light source to produce the gain in the Er-doped fiber, and a tuning-control light source to create the standing wave. This increases the number of required components for filters of this type and greatly increases their cost. Further, the wavelength of the tuning-control source must be precisely matched to that of the signal, requiring a wavelength-locking feedback loop control, adding even greater complexity. Finally, adding a new signal wavelength to increase capacity would require addition of another tuning control source in every filter, making such upgrades prohibitively costly.
Moreover, refractive-index filters split the wave from the tuning-control light source and direct each beam through both ends of the doped medium to induce the refractive-index grating. This necessitates additional optical components such as couplers, loops and polarization controllers. These extra components not only increase the cost of the filter, but also create instability of the standing wave because of the error introduced to the beams as they pass through these additional components. Since loss of coherence between the two beams will destroy the signal output, this instability severely limits the robustness of the system.
In addition to noise, power control is a critical issue in lightwave communications systems. Systems using multiple wavelength channels can suffer severe penalties if the channel power levels vary too widely. Slight wavelength dependence of the optical amplifier gain can lead to such imbalances after an amplifier chain. Therefore, automatic adjustment of channel levels, known as channel equalization, must be performed periodically.
One method of channel equalization is to separate the wavelengths, measure the power of each one, and adjust the gain or loss experienced by each channel before recombining them. Equipment to perform this chanel-by-channel adjustment, however, is expensive and can degrade the signal-to-noise ratio of the channels.
A twincore erbium doped fiber amplifier (TC-EDFA) with a channel equalization of 1 decibel (dB) is discussed in a conference report titled "Channel power equalizing WDM link incorporating twincore erbium doped fibre amplifiers" by Oliver Graydon et al., Summaries of the papers presented at the topical meeting Optical Amplifiers and Their Applications, Monterey, Calif., Jul. 11-13, 1996. When multiple channels are launched into one core of a pumped Er-doped twincore fiber, the channel powers couple from core to core along the length of the fiber. This beatlength, however, is wavelength dependent and thus the channels become periodically spatially separated. This decoupling of the channels gives the Er-doped fiber amplifier inhomogeneous saturation characteristics and allows the channels to saturate the gain to some degree independently. As a result weaker channels will receive more gain than stronger channels. This intrinsic equalizing effect keeps the channels propagating with constant powers along the TC-EDFA cascade.
A problem with the Graydon equalizer is that it requires the use of dual-core fiber. Dual-core fiber requires precisely controlled coupling, making the Graydon equalizer difficult to manufacture. Further, it is difficult to eliminate the undesired polarization and wavelength dependence requirements.
Accordingly, it becomes readily apparent there exists a need for a method apparatus for implementing a self-tuning filter and a self-adjusting channel equalizer that is robust and solves the above-discussed problems.